Nervous to tell my first born

He is 9 years old such an amazing sweet laid back kid. Whenever we bring up the topic of him having a brother or sister he literally will say NOOOO!!!!! he is perfectly happy being the only child but I’m 12 weeks and my family knows but they know not to mention it around him. We wanna tell my husbands family but it’s a bigger family and they are not as considerate and I feel like it may slip out so I need to tell my son before we tell my husbands family. I was thinking of getting him a nice big brother book for Christmas since he’s way older and understands a bit more. Plus I should know the gender by then so I hope it’ll make it more exciting. I’m a nervous wreck I don’t wanna hurt his feelings 😭